Test target assemblies provide various target signals at visible, near-IR and far-IR (thermal) wavelengths for calibrating and testing missile launcher sights. The conventional assembly contains an emissivity target plate in the front side and illumination sources placed behind the plate. The emitted test target light is collimated by a collimator assembly for presentation to one or more of the missile launcher sights. The emissivity target plate has a glass substrate coated with a thin layer of chrome on the front surface; the target patterns are created by etching through the chrome layer to expose the glass surface. Due to the different emissivities of chrome and glass, these target patterns provide simulated differential temperature targets, e.g., for the thermal (night) sight of a missile launcher.
Some targets are required to emit not only thermal signals but also visible and near-IR signals needed for testing the day sight and the tracker of a missile launcher. The visible and near-IR signals are provided by illuminating from the back side of the target with appropriate sources.
In one known test target assembly, the central target pattern is a small aperture of 0.0087" diameter which requires illumination from the back side with visible (0.65 .mu.m), 0.9 .mu.m and 1.3 .mu.m wavelength sources. The central boresight target of this test target assembly is illuminated with three LEDs through a coupler including an optical fiber bundle mixer and a homogenizer. The light from three different LEDs is coupled into three thin input fiber bundles of the mixer. The other end of the mixer is a thick output fiber bundle formed by mixing the individual fibers from the three thin bundles. A homogenizer is used to further mix the light from the three different LEDs. The homogenizer is placed directly behind the back side of the target plate without optical relay. The total coupling efficiency of this test target assembly is relatively low. Moreover, in practice, the uniformity of mixing individual fibers in the currently available optical fiber bundle mixers is quite poor; as a result, the irradiance uniformity from the central boresight target is not satisfactory and the boresight test accuracy is reduced. Placing the homogenizer directly behind the target plate without optical relay limits radiation angular spread from the center boresight target aperture. Narrow angular spread results in poor test target performances, i.e., higher boresight error, and difficulty for the missile sight to track the test target. To correct the tracking, the target assembly has to be uniquely designed by pointing the target toward the tracker, which increases assembly difficulty.